Is Dark Energy Propelling the Accelerating Expansion of the Universe?

[ibysaiyan]

Hi all,
I would like to share this interesting article :
http://www.bbc.co.uk/news/science-environment-13462926"
Einstein was right , does this mean we have confirmed the existence of some 74% our universe but not sure of the whole mechanism ?
EDIT:I was just discussing about this discovery with a friend ,isn't this disappointing in a way, for this ever increasing expansion would eventually lead us to a spot where discovering other phenomenon be beyond our reach ?
Regards,
ibysaiyan

 

[Lost in Space]

The results seem to confirm the standard cosmological model and Einstein's cosmological constant, although dark matter and dark energy still aren't fully understood. If the universe is open ended and goes on expanding forever as the evidence seems to suggest, does this mean that infinity has a starting point, i.e. the Big Bang, or is the Big Bang just part of some even greater process of continuity along the lines of what Fred Hoyle thought?

 

[e2m2a]

The results seem to confirm the standard cosmological model and Einstein's cosmological constant, although dark matter and dark energy still aren't fully understood.

Consider this quote from Wikipedia, under dark energy: "Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously...(emphasis added)"

Question. What's wrong with the cosmological constant as an explanation for the acceleration of the universe? Answer. Newton's second law.

Energy in a given volume of space equates to Newtons per meters squared, which is pressure. (Do a dimensional analysis of energy over volume and you end up with Pascals.)

In order for energy in space to accelerate galaxies, matter, etc., there must be a net force acting on the matter. (This is Newton's second law.) In order for pressure to effect an acceleration there must be a pressure differential. If the the cosmological constant predicts a "constant energy density filling space homogeneously", then there would be no pressure differential due to this energy in space, no net force, and no acceleration.

 

[cepheid]

Hi all,
I would like to share this interesting article :
http://www.bbc.co.uk/news/science-environment-13462926"
Einstein was right , does this mean we have confirmed the existence of some 74% our universe but not sure of the whole mechanism ?

I posted about this here: https://www.physicsforums.com/showthread.php?t=500187

including a link to the first paper that is mentioned in that BBC article (the one about baryon acoustic oscillations, or BAOs). Maybe the second paper is more impressive, but it seems to me that this doesn't quite live up to the media hype. It seems like what the first paper says is that using their data on BAOs, they can come up with constraints on the values of the cosmological parameters that are consistent with those obtained from CMB and supernova type Ia datasets, all of which point to the existence of dark energy. So, given that there are already two additional classes of observational data sets (CMB and Type Ia Supernovae) that independently favour cosmological models with non-zero "Lambda" or "cosmological constant" terms, how does this data set constitute "the first independent confirmation of Dark Energy" like that CBC article I linked to claims? Isn't that overstating things? (To be fair, the BBC article just said that this was AN independent confirmation of dark energy, which was less bold a statement than the CBC article.)

EDIT:I was just discussing about this discovery with a friend ,isn't this disappointing in a way, for this ever increasing expansion would eventually lead us to a spot where discovering other phenomenon be beyond our reach ?
Regards,
ibysaiyan

I saw a lecture by Lawrence Krauss in which he discussed this -- in the far future, none of the pieces of observational evidence (redshifts, CMB, etc) that form the pillars of the standard cosmological model will be available. Everything outside of our local group will be redshifted away. So, as far as any intelligent individuals in the far future will be able to determine, they'll think they're living in an infinite, static universe! He said more things as well, that were even more depressing (something to do with entropy and the decreasing ability of the universe to support life), but I don't remember the details.

 

[cepheid]

Consider this quote from Wikipedia, under dark energy: "Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously...(emphasis added)"

Question. What's wrong with the cosmological constant as an explanation for the acceleration of the universe? Answer. Newton's second law.

Energy in a given volume of space equates to Newtons per meters squared, which is pressure. (Do a dimensional analysis of energy over volume and you end up with Pascals.)

In order for energy in space to accelerate galaxies, matter, etc., there must be a net force acting on the matter. (This is Newton's second law.) In order for pressure to effect an acceleration there must be a pressure differential. If the the cosmological constant predicts a "constant energy density filling space homogeneously", then there would be no pressure differential due to this energy in space, no net force, and no acceleration.

General relativity (GR) supersedes Newtonian mechanics when it comes to the dynamics of the expansion of the universe. And GR says some things that aren't so obvious or in line with our intuition (or at least mine). Part of the problem with your reasoning is that although pressure and energy density have the same dimensions, they are not necessarily equal to each other (e.g. for photons, P = 1/3 rho, and for non-relativistic matter, a negligible pressure exists, and we take P = 0). The relationship between pressure and density for a substance is called the equation of state. What does the equation of state of a substance have to look like if that substance (e.g dark energy) is to cause accelerated expansion? To answer that, take a look at the second equation here:

http://en.wikipedia.org/wiki/Friedmann_equations#The_equations

which is called the Friedmann acceleration equation (and is derived from the Einstein field equations of GR). The acceleration of the expansion is described by second derivative of the scale factor a. (The scale factor is $a$ and its second derivative is $\ddot{a}$). You can see that (taking c = 1 for convenience) in order for the acceleration to be positive, it must be true that $P < -(1/3)\rho$ where P is pressure and rho is energy density. The Friedmann acceleration equation just gave us the constraint on the equation of state that we needed. Now, another thing that is totally not obvious is that if something has a constant energy density, then it has a negative pressure given by $P = -\rho$, which satisfies the above criterion for accelerated expansion. Again, this is really not obvious, but I go over it here:

https://www.physicsforums.com/showpost.php?p=3264800&postcount=24

 

[e2m2a]

General relativity (GR) supersedes Newtonian mechanics when it comes to the dynamics of the expansion of the universe.

Here, we have to assume the universe is expanding. Friedmann's 3 famous models based on GR, are mathematical models, not laws of the universe. We cannot elevate his models to be axiomatic conditions of the universe. This would not be science, but chalk board metaphysical speculation. No one can prove his expansion model.

GR may supersede Newtonian mechanics in some cases, except in the weak-field limits, but does it supersede other known laws, such as the second law of thermodynamics-- entropy? How does Friedmann's equations "work around" the law of entropy?

Again, I am assuming GR does not throw away the concept of force completely. Yes, gravity in GR may be considered a "fictitious" force, but what of electromagnetic-forces-- Maxwell's Equations pertaining to the force exerted by E-fields on charged particles? Special relativity did not modify Maxwell's equations, only Newtonian mechanics. Does GR modify Maxwell's equations?

My point is this. This dark energy must be in the form of electro-magnetic energy that fills all of space, unless its a new "exotic" form of energy, made of something else. This energy must be comprised of photons, each with momentum equal to h/lambda. These photons must transfer part of their momentum to "solid" objects upon collision. Its these collisions that affect a net force on an object or we would have to say GR renders the Poynting vector irrelevant.

If GR washes this all away, then you don't need dark energy to explain the putative expansion of the universe; it expands because Friedmann's GR-based equations imply the universe is expanding without a "mechanical" explanation. Another words, you don't really need causality here.

If however, you believe there must be a "force" responsible for the acceleration of the universe, than that implies the energy density of space can't be homogenous or isotropic, else there would be no force differential due to photon collisions. But, given a volume of space with this non-homogenous density, entropy would require that this energy density imbalance would transist to its most probable micro-state, a state of energy density equilibrium. For this energy to exist always in an unbalanced energy density state, given the eons of time the universe has existed, would be highly improbable. Or, does GR supersede the laws of entropy too?

 

[Drakkith]

Can someone answer me this? Does the expansion of the universe, in the sense that every point in space is getting further from every other point, require any force or something? I could see the expansion of the physical objects in the universe, but if we are talking about space, then I am confused.

 

[Lost in Space]

Can someone answer me this? Does the expansion of the universe, in the sense that every point in space is getting further from every other point, require any force or something? I could see the expansion of the physical objects in the universe, but if we are talking about space, then I am confused.

It's said that the expansion rate is increasing over time. Before this was discovered, three options existed. 1) That space would expand forever. 2) That an equilibrium would be reached. 3) That the gravity from mass in the universe would be enough to reverse expansion and create a 'Big Crunch'.

On 'weighing' the universe it was found that there wasn't enough mass to reverse the expansion (this was before dark matter was discovered). It still came as a surprise however, that the expansion rate was increasing. So even with dark matter there still is not enough mass to cause a 'Big Crunch'. Whatever the 'force' is that is responsible for expanding space, it is enough to overcome the mass in the universe and seems to be unaffected by gravity, indeed it's almost like antigravity in its ability to repulse but is far stronger than gravity (except perhaps inside black holes?).

Space, or more specifically the vacuum, is said to be made of virtual particles constantly coming into existence and going out of existence. These must be a manifestation of energy? But if these virtual particles are increasing in number, which they must be if space is expanding, where are they coming from or is the energy that produces them infinite? Are they really being 'created', or are they somehow leaking into this universe from another place? And why should the rate of expansion of space increase at all? Up until a few years ago the only increase of the rate of expansion was hypothesised with the inflation theory of the early universe.